Cathode ray tube



April 24, 1945- v. J. scHAEFER 2,374,311

CATHODE RAY TUBE Filed June 2l, 1944 Inventor: v Vincent J. Schaefer Patented Apr. 24, 1945 CATHODE RAY TUBE Vincent J. Schaefer, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 21, 1944, Serial No. 541,401

Y which is visually responsive to the impingement 4 Claims.

The present invention comprises an improved cathode ray device, this application being a continuation-impart of my prior application, Serial No. 400,069, led June 27, 1941.

In accordance with one of the features of my invention a smooth reflecting layer of metal is provided on a relatively granular surface such as the surface of a fluorescent screen contained in a cathode ray tube which may be used for television pictures or other purposes. cordance with my invention such a screen is provided with a. smooth, preformed, light-reflecting metal coating. Such a reflecting coating enhances the light output, prevents light and secondary electrons generated in the screen from being projected back into the free space of the cathode ray tube, prevents ions formed in the discharge space from reaching the screen and conducts away undesirable electric charges from the screen.

By the present invention a smooth, uniformly thick and strongly adherent metal coating of appropriate characteristics may be obtained by a procedure which is described and claimed in my application, Serial No. 400,069, led June 27, 1941, and is explained herein connection with the accompanying In the drawing Fig. 1 shows a partially sectionalized cathode ray tube embodying the present invention; Fig. 2 illustrates a particular step in the practice of the invention; Figs. 3, 4 and 5 are enlarged fragmentary sectional views showing a fluorescent cathode-ray tube screen structure in successive stages.

In Fig. 1 is shown a television type of cathode ray tube having an envelope which comprises an enlarged bulbous portion I and an elongated shaft portion 2. The latter portion contains an electron gun 3 for generating a stream of electrons adapted to be projected longitudinally oi the envelope. As the structure of a so-called electron gun is well understood, it will not be herein described. A conductive coating 4 (e. g., an aqueous suspension of graphite) is applied to the inner surface of the envelope. The coating I extends to within a short distance of the electron gun and constitutes the anode or accelerating electrode by means of which an appropriate axial field may be created within. the envelope. The electrodes thus are constructed and oriented to project electrons upon the end of the envelope remote from the electron gun 3, this remote end being in the form of a flattened transparent window 5. It is provided on its inner surface with a layer or screen 6 of a substance In acof electron (i. e., cathode rays). Electron deecting means, illustrated diagrammatically as defiecting coils 1 and 8, may be provided adjacent an intermediate portion of the tube for the purpose of causing the electron beam to scan sequentially the various elemental areas of the iluorescent screen so as to develop a visible pattern or picture on the screen. The substance forming the screen 6 may consist of a fluorescent material, such as willemite (zinc silicate), which emits light when subjected to cathode rays, or it may consist of a so-called dark trace material, for example, an alkali halide, on which When externally illuminated a dark trace or image may be produced by cathode rays.

It is obvious that a portion of light developed by the screen 6, if not impeded, would be projected back into the interior of the tube rather than forward through the window 5. The light which is thus projected back in a sense, is lost. In any event light shining into the tube tends to produce objectionable internal reections. As a consquence of my invention as will be hereinafter more fully explained not only`is the undesired transmission of light to the interior parts of the cathode ray device prevented but by reflection of the light the output eiciency of the screen is enhanced by the reflection of this portion of the light in a forward direction.

In Fig. 2 hereof, there is shown the rst step of a method by which a cathode ray screen may be provided with a metallic coating of the desired character. This gure shows the bulbous portion I of a cathode-ray tube =envelope before the same has been joined to the shaft portion of the envelope. In the condition illustratedthe endwall of the envelope portion l is assumed to be provided with an adherent coating of liiuorescent material or dark trace material, as indicated at 6, and which in the completed form of the tube is intended to function as a screen.

According to the present invention, the 'end- Wall of the tube on which an adherent layer 6 has been deposited is placed in contact 'with a cooling plate S having a surface which conforms to the outer surface of the window 5. The screen may have a rough or granular surface. The cooling plate is brought to a temperature slightly above the freezing temperature of a chosen liquid (water, for example) by circulating a suitable refrigerant through ducts l0 provided in the body of the plate. After the tube structure has been adequately cooled by this means, vapor of the chosen material which may readily be congealed is introduced into the interior of the envelope preferably as a vapor by a nozzle indicated at II. The vapoir whidh reaches the end-wall of the tube will be condensed, because of the lower temperature at which the wall is maintained, thus filling the interstices which separate the various granular particles of the screen 6. The surface then is lowered to a temperature below the freezing point of the liquid to solidify the condensed lm. In this way the irregularity by which the screen is characterized initially will be eliminated. The screen in combination with the adhering deposit of ice, or other frozen material (indicated at I2 in F153), will present a smooth surface exposed to the interior of the envelope I.

In the foregoing, reference has been made to the use of water as the freezable liquid to be emf ployed simply because this substance is ordinarily available and can be caused to pass from its solid to its vapor stage within a range of temperatures practically attainable. It should be lmderstood, however, that other normally liquid or gaseous substances may be used in this connection. For example, benzene, acetone, or alcohol may be employed.

After this step is completed, one may iiow over the surface of the frozen screen a very thin layer of a film-forming resin dissolved in a volatile solvent and introduced by spraying or otherwise. In this latter connection one may employ numerous resinous materials including, among others, polystyrene, cellulose esters, such as nitro cellulose and cellulose acetate; resinous compoimds of the polyvinyl type. as exemplified by plasticized polyvinyl halides; by the copolymer of vinyl chloride and vinyl acetate; or by one of the reaction products of an aldehyde with a hydrolyzed polymerized vinyl ester. 'I'he last named reaction products are described full in Reissue Patent 20,430, Morrison et al., and are designated generally as polyvinylal resins." A typical polyvinylal resin and one which is considered especially suitable for the present invention is that which is prepared by condensing formaldehyde with the product of hydrolysis of polyvinyl acetate, the resultant resin being conveniently designated as polyvinyl formal.

'I'he solvent to be used will vary with the particular resin selected. In the case of polystyrene carbon tetrachloride may be used as an approprlate solvent; with polyvinyl chloride one may use monochlortoluene; with polyvinyl formal, chloroform or ethylene diclxloridel etc.

After the particular resinous material selected has been spread to the desired thickness, the resin should be allowed to set by evaporation of the solvent, this latter step preferably taking place while the tube wall is maintained at a relatively low temperature. Under these conditions, the resultant resin iilm, which is preferably of a thickness between 200 and 10,000 Angstrom units, will have an essentially smooth surface as indicated at I3 in Fig. 3. After the resin has set (i. e., by evaporation of its solvent) the temperature of the assembly may be slowly raised in order to melt the liquid on which the resin is superimposed and finally to cause the liquid to be evaporated by passage in the vapor phase through the resin lm.

The resin lm may next be coated, as illustratedin Fig. 4 (e. g., by an evaporating process), with a thin deposit Il of a highly reflecting metal, for example, silver or aluminum. Fig. 4 shows in successive layers the screen I. tl resinas'un ouslm I3 andametalliccoating Il. Thelatter may have a thickness of about 500 to 1200 A. 'lhe layers shown are ailixed to the inner surface of the glass window of the tube. Due to the manner of its formation and, more specifically, to the nature of the base on which it is formed, the metal coating Il will be uniformly thick and of a highly reflecting nature. In some cases the reiiecng metal layer may be deposited directly on the congealed water, benzene, etc., illling the voids between the particles of the screen 6, thus dispensing with therin layer.

The metal layer I I during evaporation will make electrical contact with the conductive coating l, which is positively charged during the operation of the device. The deposition of a metal layer on the interior surface of the screen l may beutilizedtowhollyorpartly dispensewiththe application of a conductive coating of graphite on the interior of the bulbous portion I. In any event the coating 4 and the coating I3 will overlap to some extent.

During the usual bake-out of the cathode ray tube in its process of manufacture following the application of the described layers, the resin layer I3 is largely or entirely dissipated by evaporation. In Fig. 5 is shown an enlarged sectional view of a portion 0f a cathode screen from which the volatilizable resin layer has been thus entirely removed. .The preformed metal layer Il is thus superimposed on the projecting parts of the irregular screen without Dartaking of all of its irregularities as would be the case were the metal layer evaporated directly upon the screen. IA temperature of about 500 C. is suitable for the dissipation of the resin. The resin film due to its thinness and transparency has so little effect on the functioning of the metal film I4 that it is notrequiredinallcasestoremovetheresiniilm in order that the cathode ray tube may flmction as intended.

The metal layer Il, in addition to usefully refleeting light in a forward direction, carries away undesired sticking potentials or electric charges from the screen due to its electrical connection with the positive electrode l.

The thinness of the reflecting metal layer (the dimension of which is exaggerated in Figs, 4 and 5) does not prevent the high velocity electrons from the cathode or electron gun 3, from reaching the screen i. Positive ions, however, are intercepted thereby and thus prevented from deleteriously affecting the screen.

While the invention has been described by reference to particular uses thereof, it will be understood that other applications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent uses as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to'secure by Letters Patent of the United States is: v

1. A cathode ray tube comprising an envelope, cooperating electrodes therein which are constructed and oriented to project a beam of electrons upon a window portion of said envelopea screen of electron-responsive material located upon the inner surface of said window and exposed to said electron beam, an electron-pervious conductive preformed light-reflecting layer of metal superimposed on said screen on the inner surface of said screen which is exposed to said beam said layer having a smooth surface presented toward said screen.

conductive lm of light-reflecting metal superimposed on said layer.

4. A cathode ray screen comprising a foundation of light-transmitting glass, a, coating of electron-responsive material on said foundation, said coating having a rough surface, a superimposed electron-pervious, light-transparent layer of volatilizable resin having a substantially uniform thickness and supporting a smooth conductive 10 film of metal coating said resin layer.

VINCENT J. SCHAEFER. 

